Translation:He loves himself.
No, due to the nature of the French language there is (and should not be) no audible difference between the two. French is a pretty hellish language ( :P ) and one of its base ideologies is to be guided by context, that is why, unlike English, solitary sentences do not make much sense.
In general, when ils is elided it makes more of a z sound, though I am not sure that's true when elided with s, in fact i doubt it. Another subtle difference is that the -ent words have (IIRC) slightly more stress on the final constant. So il aime has a very soft m whereas ils aiment is more pronounced, like 'ahmm'.
it's not a short form. You don't put direct and indirect object pronouns after the verb, they go before the verb. Also, lui is the indirect object pronoun.
So you could say "Je lui parle" (I am talking to him) but "Je le aime" (I like him) which is contracted down to "Je l'aime"
This would be translated to he loves himself, because when you use "aimer" with a pet or a human, it means to love. To write he likes himself I think you would have to write "il s'aime bien". When you add the "bien" afterwards, it means like. This is also how you would write he likes ___ (any animal or human).
It's fairly common in French when direct or indirect pronouns are used. https://www.thoughtco.com/french-indirect-objects-1368865 might help.
Shouldn't be. Should be either Il s'aime or Ils s'aiment but not mixing the two. Hope you put a report in. http://la-conjugaison.nouvelobs.com/du/verbe/s_aimer.php for conjugation of s'aimer.
Once I answered
He loves himself and it was rejected in favor of
He loves him.
This time I answered
He loves him and it was rejected in favor of
Hi loves himself.
Some royal person decides when you are right and when you are wrong. There is not necessarily any logic behind that.
He likes himself, refusé. Oh, les surdoués de DL, vous devez bien être au courant qu'en français on a le même mot pour like et love, et que nous ne nous sentons pas obligés de préciser "bien" à tout bout de champs ! Il faudrait vous décider à être un poil plus souples de temps en temps histoire de nous éviter ce type de refus légèrement abusif...